Crossword Genius

... losing head, affair leads to marriage (8)

Ross

I believe the answer is:

alliance

'marriage' is the definition.
(I've seen this before)

'losing head affair' is the wordplay.
'losing head' means to remove the first letter.
'affair' becomes 'dalliance' (I've seen this before).
'dalliance' with its initial letter taken away is 'ALLIANCE'.

'leads to' acts as a link.

(Other definitions for alliance that I've seen before include "Union, association" , "Marriage - union" , "Coalition or union" , "Bond" , "Federation" .)

I've seen this clue in The Telegraph.
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